1. Field of the Invention
This invention relates to improved devices for providing hearing protection from exposure to sounds that are intense enough to risk hearing damage.
2. Background of the Prior Art
It is well documented that repeated or prolonged exposure to sounds of sufficiently high sound pressure level (SPL) will cause temporary or permanent hearing loss. A good summary of research in this field can be found in the book Noise & Hearing Conservation Manual, Fourth Edition edited by E.H. Berger, W.D. Ward, J.C. Morrill, and L.H. Royster and published by the American Industrial Hygiene Association, Akron, OH (1986). The chapter "Auditory Effects of Noise" written by W. D. Ward is particularly recommended. The application of hearing protection in the form of earmuffs or earplugs is commonly employed to reduce the SPL reaching the ear, thereby preventing hearing loss. ANSI standard S12.6-1984 (American National Standards Institute, N.Y., N.Y.) describes the accepted Real Ear Attenuation at Threshold method for measuring the amount of reduction in SPL reaching the eardrum that results from the application of a particular earmuff or earplug.
Earplugs and earmuffs suitable for preventing hearing loss are widely available. An earplug that is widely used for hearing conservation in industry is the slow-recovery foam earplug described in Reissue Pat. No. 29,487 Reissued Dec. 6, 1977, and manufactured by the E-A-R Division of Cabat Corporation. This plug, trademarked as the E-A-R plug has been demonstrated to be highly effective in preventing hearing loss as summarized by J.D. Royster and L.H. Royster in their chapter "Audiometric Data Base Analysis" in the aforementioned book. When properly inserted, the E-A-R PLUG provides approximately 35 dB of attenuation at low frequencies and approximately 45 dB of attenuation at high frequencies. Similar high frequency attenuation values have been reported for other well-sealed earplugs including custom earmolds fabricated specifically for hearing protection. Although such large attenuation values are required for protection against exceedingly high sound pressure levels such as near jet aircraft or the like, more common working environments may require only 10 to 20 dB of attenuation in order to be safe.
Earplugs of the foregoing type have had certain inherent disadvantages. In applications where the user wants or needs to hear clearly, the high-frequency attenuation provided by the foregoing earplugs is excessive, resulting in a muffled sound. One example is provided by the mechanic in a moderately noisy factory who must listen to the sound of the machinery in order to monitor proper operation or diagnose the likely source of malfunction. Another is the musician in a confined rehearsal or performing space who must listen to the sound of the other musicians in order to play properly. It is well known that individuals in both situations routinely refuse to wear hearing protection. A recent news item in the Jan. 18, 1988 issue of Fortune magazine reported a Colorado high school district edict requiring band directors to wear hearing protection because of worker compensation cases, and further reported the incredulous refusal of the band directors to do so: "It's difficult enough to provide music on a competent level with your ears wide open ..." one was quoted as saying.
Earplugs providing less high frequency attenuation have been known. Extensive real ear attenuation measurements on 21 brands of earplugs were reported by J.B. Tobias, for example, in FAA report FAA-AM-73-20 (Washington, D.C., 1973). In all cases, earplugs with reduced attenuation at high frequencies provided little or no low frequency attenuation, again resulting in an unnatural sound somewhat similar to the sound of a high-fidelity record player with the bass turned up and the treble turned all the way down.
Most recently, high-fidelity earplugs suitable for use with custom earmolds have been introduced by Etymotic Research, Inc., licensed under a patent application to be filed by Elmer Carlson. These earplugs provide a uniform attenuation of approximately 15 dB across nearly the entire audible frequency band, from 20 Hz to at least 10 kHz, as confirmed by applicants own measurements. These earplugs include:
1. A flexible diaphragm as a series compliance element to provide a pressure divider with the compliance of the earcanal volume, resulting in a uniform low frequency attenuation,
2. An internal acoustic sound tube as a mass reactance element to series resonate with the compliance of the earcanal volume at approximately 2700 Hz, preserving the relative frequency response characteristics of the sound-field-to-eardrum-pressure transfer function of the normal external ear which has an approximately 15 dB peak at approximately 2700 Hz,
3. An internal series damping resistance element to limit the amplitude of the aforementioned resonance peak at 2700 Hz to approximately 15 dB, further preserving the relative frequency response characteristics of the sound-field-to-eardrum-pressure transfer function of the normal external ear which has an approximately 15 dB peak at approximately 2700 Hz, and
4. A distributed acoustic network to provide a low impedance shunting of acoustic energy at approximately 8000 Hz where the earplug attenuation would otherwise be reduced to a low value because the distance between the sound outlet of the earplug and the eardrum is nearly a half wavelength resulting in the formation of a half wave resonance condition in the earcanal at that frequency.
Although the Carlson earplug represents an important and substantial contribution, it has limitations which have not been recognized. The cost of manufacture thereof is relatively high, precision manufacture of the various acoustic elements being required in order to obtain the desired uniform attenuation frequency response. An additional limitation of the Carlson earplug is that it requires use of a precise and relatively large diameter and short length internal sound tube in the accompanying custom earmold. Depending on the sound tube length that is chosen, internal diameters of 3 to 5 mm are required for the sound tube in order to obtain the value of mass reactance needed for proper operation of the Carlson earplug. Such a requirement is not readily compatible with the use of a slow recovery foam eartip in order to make a "universal" ready-to-use version; it is found that a tube with about 2 mm inner diameter is the largest practical tube for use in a "universal" eartip. For example, a commercially available Etymotic Research ER3-14 eartip used with the Etymotic Research ER-3A audiometric earphone for hearing testing, uses a tube with a 1.93 mm internal diameter. In the ER3-14 eartip, one end of the tube is inserted into a central opening of a slow recovery foam member which has a diameter of about 18 mm and a length of about 12 mm, the tube having an opposite end coupled through a longer tube to the audiometric earphone.
A further limitation associated with the relatively short sound tube required with the Carlson earplug is that the eartip will itself also be naturally short, resulting in a seal to the ear close to the entrance of the earcanal. As a consequence, the Carlson earplug will be susceptible to a large occlusion effect as described in the article "Zwislocki was right...A potential solution to the `hollow voice` problem (the amplified occlusion effect) with deeply sealed earmolds" by M.C. Killion, L.A. Wilber, and G.I. Gudmundsen (Hearing Instruments, January, 1988). Thus the user will experience his own voice as having a "hollow sound" due to the unusually large eardrum SPL's that will be developed at low frequencies whenever he talks, and the user will be unable to understand other people talking due to the masking effect of the unusually large eardrum SPL's that will be developed whenever he chews corn chips or other crisp foods.